Anatomy of Inner Ear

Anatomy of the Inner Ear

The inner ear, also known as the labyrinth, is a key sensory structure responsible for hearing and balance. It consists of:

• A bony labyrinth
• A membranous labyrinth
• Two fluids: endolymph (inside the membranous labyrinth) and perilymph (between bony and membranous labyrinth)

🏗 Components of the Inner Ear

The inner ear is divided into:

• Vestibule
• Semicircular Canals (3)
• Cochlea

🦴 Bony Labyrinth

🔹 Vestibule

• Central part of the labyrinth

• Lateral wall: contains the oval window

• Medial wall: contains two recesses:

  • Spherical recess
  • Elliptical recess

What does the Spherical and Elliptical recess lodge in it?

• Aqueduct of vestibule: transmits the endolymphatic duct

• Receives 5 openings from the semicircular canals

🔹 Semicircular Canals

• 3 canals: lateral, posterior, superior

• Lie at right angles to each other

• Each has:

  • Ampullated end (opens into vestibule)
  • Non-ampullated end (some join to form a common crus)

🔹 Cochlea

• 2.5 turns around the central bony axis: modiolus

• Houses 3 compartments:

  • Scala vestibuli (above)
  • Scala tympani (below)
  • Scala media (central) — contains endolymph

• The helicotrema at the apex connects scala vestibuli and tympani

• Promontory on the medial wall of the middle ear = basal turn of cochlea

💧 Fluids of the Inner Ear

• Endolymph: inside membranous labyrinth (scala media)
• Perilymph: between bony and membranous labyrinth (scala vestibuli & tympani)

Check out my post on Inner ear fluids to learn more about Perilymph and Endolymph.

🎼 Functional Anatomy of the Cochlea

• Scala media (membranous cochlea) is triangular in cross-section:

  • Basilar membrane → houses Organ of Corti
  • Reissner's membrane → separates from scala vestibuli
  • Stria vascularis → lateral wall; secretes endolymph

• Sound conduction:

  • Stapes footplate → scala vestibuli → helicotrema → scala tympani → round window

  • Pressure differences vibrate basilar membrane

  • Stimulates Organ of Corti → transduces mechanical to electrical signals

🧬 Membranous Labyrinth

🔸 Ductus reuniens: connects cochlear duct to saccule
🔸 Utriculosaccular duct: connects utricle and saccule
🔸 Endolymphatic sac: surgically important in Ménière's disease

🩸 Blood Supply of the Inner Ear

Arterial Supply — via Labyrinthine artery (Branch of AICA or sometimes Basilar artery)

• Anterior Vestibular Artery → utricle, superior & lateral semicircular canals
• Common Cochlear Artery →
  • Main Cochlear Artery → ~80% cochlear supply
  • Vestibulocochlear Artery →
    • Cochlear branch → ~20% cochlear supply
    • Posterior Vestibular Artery → saccule, posterior semicircular canal

Venous Drainage:

• Internal auditory vein
• Vein of cochlear aqueduct
• Vein of vestibular aqueduct → Drain into inferior petrosal sinus and lateral sinus

📌 Clinical Highlights

• Footplate of stapes is closely related to the saccule.

  • In Ménière's disease, a distended saccule may press against it.

• Organ of Corti, macula, and crista ampullaris are the key sensory zones.

• Von Békésy's theory explains basilar membrane motion and wave propagation.

📝 Summary Table